FIG. 6 is a cross-sectional view of a prior art photodetector for detecting infrared light that is described in Japanese Published Patent Application 1-246878. The photodetector of FIG. 6 includes a light absorbing layer 1 of cadmium mercury telluride (Cd.sub.x Hg.sub.1-x Te) disposed on a cadmium telluride (CdTe) substrate 5. A doped region 3 at a surface of the Cd.sub.x Hg.sub.1-x Te layer 1 remote from the substrate 5 forms a pn junction 4 within that layer 1. An electrode 7 is in contact with the doped region 3 and an electrode 8 is in contact with the undoped portion of the light absorbing layer 1. Spaced apart regions 10 of Cd.sub.x Hg.sub.1-x Te are disposed on the surface of the Cd.sub.x Hg.sub.1-x Te light absorbing layer 1, partially overlying respective doped regions 3, and are electrically insulated from the electrodes 7 and 8 by intervening insulating layers 11.
Typically, infrared photodetectors of the type shown in FIG. 6 are operated at a reduced temperature, such as liquid nitrogen temperature (77.degree. K.), for detecting infrared light in the ten micron band, i.e., light having wavelengths from approximately eight microns to approximately twelve microns. Incident infrared light 9 passes through the wider band gap substrate 5 and reaches the narrower band gap light absorbing layer 1 where the light is absorbed, thereby producing charge carriers that are collected as a photocurrent through the electrodes 7 and 8.
FIGS. 7(a)-7(f) illustrate a method of making the prior art photodetector of FIG. 6. As shown in FIG. 7(a), a p-type Cd.sub.x Hg.sub.1-x Te light absorbing layer 1, where x=0.2, is grown on a substrate 5 of CdTe. In addition, a thin p-type semiconductor layer 10 of Cd.sub.x Hg.sub.1-x Te where x=0.3, is grown on the semiconductor layer 1. The semiconductor layers 1 and 10 are grown by conventional techniques, such as metal organic chemical vapor deposition (MOCVD) or liquid phase epitaxy (LPE). Because the semiconductor layer 10 has a proportionally larger cadmium content than the light absorbing layer 1, it has a wider energy band gap than the layer 1.
As illustrated in FIG. 7(b), an aperture is opened in the semiconductor layer 10 exposing the light absorbing layer 1. Thereafter, a zinc sulfide (ZnS) layer (not shown) is deposited to a thickness of about one micron on the light absorbing layer 1. An etching mask 20 having an aperture larger than the aperture in the semiconductor layer 10 and encompassing the aperture in the layer 10 is then formed by conventional photolithographic techniques.
The n-type region 3 in the light absorbing layer 1 is formed by the diffusion or ion implantation of dopant impurities into the light absorbing layer 1 to produce the pn junction 4. Because the aperture in the mask 20 is larger than the aperture in the semiconductor layer 10, a narrow n-type doped region 10a within the aperture in the mask 20 is formed in the semiconductor layer 10, as shown in FIG. 7(d). Subsequently, the mask 20 and the doped region 10a in the semiconductor layer 10 are removed to produce the structure shown in FIG. 7(e).
Finally, the device structure is completed, as shown in FIG. 7(f), by depositing the insulating film 11, opening a first aperture in the insulating film 11 at the doped region 3 and a second aperture in the insulating film 11 spaced from the doped region 3, and forming the electrodes 7 and 8 in the respective apertures.
One of the most important characteristics of any photodetector is the magnitude of the dark current, i.e., the current that flows through the biased photodetector when no infrared light is incident on the photodetector. The dark current is a kind of noise. A small dark current indicates low noise and a high quality photodetector with a high signal-to-noise (S/N) ratio. In an ideal photodetector, i.e., photodiode, the dark current is limited by the diffusion of charge carriers or the recombination of charge carriers in the depletion region at the pn junction. However, in an actual photodetector, additional currents are present that contribute to the total dark current. For example, leakage current flows along exposed surfaces of the photodetector's pn junction, increasing the dark current and decreasing the S/N ratio.
In the photodetector of FIG. 6, the wider energy band gap semiconductor layer 10 is disposed on the surface of the light absorbing layer 1, covering the pn junction at the surface of the light absorbing layer 1 in order to reduce the surface leakage current. In the specific embodiment of the photodetector described with respect to FIG. 6, the energy band gap of the light absorbing layer 1 is about 0.12 eV and the energy band gap of the semiconductor layer 10 is 0.25 eV. At an operating temperature of 77.degree. K., the surface leakage current is reduced by a factor of 10.sup.4 over a similar structure lacking the semiconductor layer 10.
FIG. 8 shows a schematic cross-sectional view of an imaging device including a photodetector having the structure of FIG. 6 and a charge transfer circuit incorporating a charge coupled device (CCD) 13. In all figures, the same reference numbers designate the same or corresponding elements. In FIG. 8, the charge transfer circuit includes a p-type silicon substrate 14 in which a plurality of n-type regions 15 are produced. An electrically insulating film 16 on the substrate 14 includes apertures providing access to the n-type regions 15 and to the substrate 14. Electrodes 17 are disposed in each of the apertures in the insulating film 16 where an n-type region 15 is present. An electrode 18 is disposed in an aperture in the electrically insulating film 16 where the p-type substrate 14 is exposed. Masses 12 of indium are disposed between and provide electrical connections between respective pairs of electrodes 7 and 17 and electrodes 8 and 18 of the photodetector and CCD 13. Infrared light 9 is incident on the rear side of the substrate 5, passes through the substrate 5 with little absorption, and is absorbed in the light absorbing layer 1, producing charge carriers that are collected by the respective electrodes 7 and 8 as photocurrents. The photocurrents are conducted to the CCD 13 and supplied by the CCD 13 as an output signal representing an image of the incident light.
In the photodetector structure of FIG. 6, the semiconductor layer 10 must be relatively thin. In addition, the requirement of growing two layers, i.e., layers 1 and 10, of different energy band gaps increases the complexity of the fabrication process. The difficulty of growing a very thin layer and controlling a change in energy band gap reduces the yield of acceptable photodetectors made by the process illustrated in FIGS. 7(a)-7(f). Moreover, in providing access through the layer 10 to the semiconductor layer 1, a non-planar surface must be prepared. The lack of a planar surface increases the probability that the insulating film 11 may peel off the layers 1 and 10 and increases the difficulty of interconnecting the photodetector structure with the CCD 13 employing the masses 12 of indium. The non-planar surface can result in the formation of voids when the indium masses are formed by vapor deposition and a lift-off step. Since the CCD 13 is attached to the photodetector bearing the indium masses 12 by compressing the photodetector and the CCD 13, it is less important that the electrodes 17 and 18 of the CCD 13 be planar than that the electrodes 7 and 8 be planar.